Maybe you can't find your favorite app on your FireTV. With this app "Apps2Fire" (Apps to Fire) you can upload every Android application from your mobile phone or tablet to your Fire TV or Fire TV stick.
You only have to enable the developer options on your fire tv and enter the fire tv ip address in Apps2Fire.
Sideloading uses the Fire TV's developer options as described in the help section of Apps2Fire. Developer options are sometimes unreliable so be patient with it. If you are facing problems switch off and on again the developer options on the Fire TV and frequently restart Fire TV.